I very much appreciate the opportunity to review this manuscript. My major comment is about the threshold used to define marine heatwaves, which is the “90th percentile of climatological temperature on each day in the baseline period of 1990-2009 using an 11-day window centered on each day”. This is a fixed baseline method. For future projections with distinct long-term warming trends, authors may consider using shifting baseline as well, e.g., Amaya et al. (2023), to separate the long-term warming signal, especially for the sea surface temperature.  Amaya, D. J., Jacox, M. G., Fewings, M. R., Saba, V. S., Stuecker, M. F., Rykaczewski, R. R., ... & Powell, B. S. (2023). Marine heatwaves need clear definitions so coastal communities can adapt. Nature, 616(7955), 29-32.

Other comments are listed below:

1. The model ability to reproduce historical SST variation has been assessed, but its ability to reproduce bottom temperature variation is not included or mentioned at all.
2. Figure 4a colorbar label suggests “percentage of summer in a marine heatwave at the sea surface in 2080-99” but this figure includes all four seasons, which is confusing.
3. Figure 4b and Figure 3 seem to provide duplicate info about marine heatwave frequency at the sea surface for annual and each season. Figure 4b seems to just plot those lines together instead of separating them in each panel.

Minor comments:

1. Line 100: IThe -> The
2. Line 103: “which used global models which” -> “which used global models that”
3. Lines 124-125: degree Celsius symbol error
4. Figure S1 caption: “north west” -> “northwest”
5. Figure 2 caption: “heat waves” -> “heatwaves”
6. Figure 3 caption: “North West” -> “Northwest”

Reviewer comments on “Seafloor marine heatwaves outpace surface events in future on the

northwest European shelf“ by Wilson et al.

The authors analyze the occurrences and characteristics of marine heatwaves (MHW) on the Shelf Northwest in a warming climate in an ensemble of downscaled climate model simulations. They follow the definition of Hobday et al. and apply a fixed baseline approach. The authors concentrate on MHWs at the surface and the seafloor and contrast the respective MHW occurrences as well as give more details on seasonal differences.

The article is well written and helps to deepen our understanding of MHW conditions in this study region and by shedding light on the differences between sea surface and sea floor and between the seasons. The majority of MHW literature so far focuses on the surface and annual MHW values.

Below are some comments and questions for consideration.

Introduction:

• I would like to see a short discussion on the choice of the emission scenario used here for the future prediction, RCP8.5. I realize that, especially in high-resolution, regional modelling it’s often the first or only one to be produced, but I think it is important to qualify that this is the high-end scenario and maybe if quantify what degree of warming could be expected in other scenarios in this region. The degree of warming determines the frequency, duration and severity of MHWs.
• I would also like to see a short discussion on the choice of MHW analysis based on fixed or moving baselines. It is discussed later in the discussion section. I would like to see it earlier and read why the authors chose to apply the fixed baseline approach.
• For this study region I would like to see a more pronounced introduction to the characteristics and causes of MHW in mid-latitudes and shallow water. A lot of MHW literature is based on open ocean and or (sub-) tropical regions. Here we have more short-lived events and high-frequency triggers for onset and decline of MHWs. Stratification plays a role too. A short description of the study region in terms of stratification would be helpful as well.

Methods:

• Would be nice to read or see a very brief summary of the model evaluation in terms of temperature and most importantly stratification.

Results:

• Lines 145-147: SST changes are described. Absolute warming at the seafloor, min, max, mean?
• Line 195 ff: Does a change of timing and length of stratification play a role?

Discussion:

• Line 266: Does the degree of variability change in the projections?
• Line 284-286: Because they are deeper or next to a deep region?
• Do lateral flows at the seafloor play a role?

Figures:

Figure 1: Colormap / color limits: is there cooling (dT < 0°C)? Hard to tell. Why would that be the case? Have the same color limit for a and b.

Figures: I would prefer different colormaps for temperature changes and MHW days respectively.

Figure 2 caption: Unit would be something like MHW days per year in % Or percentage of heatwave conditions?

Figure 3,4,7 caption similar. For me, frequency means the number of MHW events.

Figure 3: Can you explain the dip in the warming curve in the IPSL model? Unfortunately, it falls into the mid-century period in this analysis…

Figure 4b: At the current size the lines are hard to see/distinguish.

Minor comments:

Throughout:

• check for missing ° signs
• for better readability consider having 1 or 0 digits for percent numbers in the text and in tables

Line 100: typo

Line 130: sea surface temperature

Line 143: Blank space